Difluorobenzotrifluorides are useful intermediates for the manufacture of herbicides. For example, U.S. Pat. No. 4,642,338 discloses the use of 3,4-difluorobenzotrifluoride in the preparation of aryloxyphenoxy herbicides that control grassy weeds in the presence of broadleaf crops.
Conventional methods of preparing 3,4-difluorobenzotrifluoride are based primarily on diazotization routes involving a number of steps. In U.S. Pat. No. 4,642,338, for example, 3,4-difluorobenzotrifluoride was prepared by (a) treating 4-chloro-3-nitrobenzotrifluoride with KF, (b) reducing the nitro group to an amine, (c) diazotizing and preparing the fluoroborate salt and (d) decomposing to the desired product ##STR1## A similar scheme was employed by Schaefer et al. in Can. J. Chem., 57, 802 (1979).
Alternatively, Musgrave et al. in J. Chem. Soc. C, 1547 (1971) describe the preparation of 3,4-difluorobenzotrifluoride by (e) the treatment of hexafluorocyclohexadiene with trifluoromethylacetylene followed by (f) pyrolysis. ##STR2##
Although highly fluorinated aromatic compounds can be prepared from perhalogenated aromatic compounds or perhalogenated aromatic compounds containing one or more electron-withdrawing substituents by the action of alkali metal fluorides, it was believed that this reaction was of preparative interest only for producing completely halogenated compounds and that reactions between incompletely halogenated aromatic compounds and KF were accompanied by numerous side reactions and poor yields. (See, for example, Yakobson et al. in Synthesis, 652, Oct. 1976).